Healthy smokers (without COPD) have increased occurrence of airway infections compared to non-smokers, and smoking is also the major risk factor for COPD. COPD is characterized by chronic airway obstruction, decreased mucociliary clearance (MCC), mucus hypersecretion, and chronic airway inflammation. Bacterial and viral infections are the leading causes of acute exacerbations of COPD. . In Cystic Fibrosis (CF), dehydration of the airway surface liquid (ASL) leads to decreased mucociliary clearance (MCC), colonization by bacteria and frequent exacerbations of disease related to MCC failure. The CFTR-induced anomalies in ASL solute concentration and subsequent ASL dehydration are a central cause of CF lung disease. Clinical similarities between COPD and Cystic Fibrosis (CF) suggest that despite differences in pathogenesis, there are key similarities in their pathophysiology. Adenosine and purinergic control of airway hydration are likely important in both diseases, as we have observed that (like CF) COPD patients also have decreased ASL dehydration and adenosine levels relative to normal volunteers. The overarching hypothesis of this project is that smoking predisposes to decreased MCC due to ASL dehydration, partly due to decreased ASL adenosine and diminished innate host defense. In Project IV, we will test the hypothesis that smokers have decreased MCC with alterations in purine and adenosine biology by comparing these airway processes between normal volunteers and smokers. We also hypothesize that smoking will cause decreased macrophage function and bacterial colonization of the airway. We will compare our results in normal volunteers and smokers to those from similarly studied COPD (Project V) and CF (Project VI) patients, as we suspect that smoking-induced changes will mimic those in COPD. We will also examine MCC, hydration and airway responses in normal volunteers and smokers after challenge with inhaled endotoxin (a bacterial product found in tobacco smoke) and experimental viral infection to determine if MCC in smokers is less adaptive to these challenges. These aims will provide novel in vivo data on smoking-induced airway pathophysiology in humans. The medical significance of these studies is that they will firmly establish the importance of mucus clearance to maintain respiratory health and will elucidate disease mechanisms and therapeutic targets applicable for many chronic obstructive airway diseases.